1. Field of the Invention
The present invention relates to a method and an apparatus for recording information on the recording layer of a magneto-optical recording medium. In particular, the invention relates to a recording method and a recorder which make it possible to form, on the recording layer of a magneto-optical recording medium, record marks from which information can be reproduced stably regardless of mark lengths of the record marks.
2. Description of Related Art
Optical recording media such as magneto-optical recording media are known as external memories for computers etc. Magneto-optical recording media can take or cover a great quantity of data such as dynamic images and voice, and are therefore used frequently as recording media in this multimedia era. In general, bits of information xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d are recorded as the presence and absence of record marks on a magneto-optical recording medium. For example, the light intensity modulation method and the magnetic field modulation method are known as methods of recording information on magneto-optical recording media.
The light intensity modulation method involves recording information by radiating a laser beam which intensity is modulated with record information, while applying a DC magnetic field to the recording layer of a magneto-optical recording medium. While the magnetic field in one direction is always applied, the information is recorded. Therefore, in order to record new information on that part of the recording layer where old information has been recorded, it is necessary to erase the old information before recording the new one. The magnetic field modulation method involves recording information by applying a magnetic field pulsed in accordance with bits of data, while radiating a DC laser beam onto the recording layer of a magneto-optical recording medium. It is possible to record the data by reversing the direction of magnetization of (part of) the recording layer with the recording magnetic field according to each bit of information xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d. It is therefore possible to directly overwrite data. Because the DC laser beam is radiated, however, the record marks become crescent in shape and longer tangentially of the recording medium as the linear velocity increases. This is disadvantageous in processing the reproduced signals.
The optical (or light) magnetic field modulation method is known as an improvement on the magnetic field modulation method. The optical magnetic field modulation (light pulse-magnetic field modulation) method involves applying a magnetic field polarized in accordance with a record signal while radiating a light beam pulsed in synchronism with a recording clock. This method overcomes or eliminates the disadvantage of the magnetic field modulation method, and makes it possible to form minute recorded magnetic domains for high-density recording.
For densely recorded information, however, a number of recorded magnetic domains will be present within a reproducing light beam spot, and it is therefore not possible to reproduce information individually from them. In other words, the resolution of the reproducing light beam is insufficient to reproduce the information individually from the minute recorded magnetic domains. It is therefore necessary to reproduce information from minute recorded magnetic domains with a reproducing light beam spot having the conventional diameter.
As a method of solving this problem, the magnetically induced super resolution technique (MSR) is suggested in, for example, Journal of Magnetic Society of Japan, Vol. 17, Supplement, No. S1, pp. 201 (1993). Even if two recorded magnetic domains are present within a reproducing light beam spot, this technique makes it possible to reproduce information from one of the domains by masking the other out of sight to narrow the effective field of view. The use of this technique makes it possible to improve the reproducing resolution without reducing the diameter of the actual reproducing light beam spot. Even with the magnetically induced super resolution technique, however, the strength of the signal reproduced from each of the magnetic domains does not change, and therefore the C/N ratio of the reproduced signal is still low.
The inventor has disclosed in International Publication No. WO98/02878 a magneto-optical recording medium including a domain enlarging and reproducing layer and a recording layer which lie on a substrate. During reproduction, the minute magnetic domains on the recording layer are transferred individually to the reproducing layer while a reproducing magnetic field is applied to the reproducing layer so that the domains transferred to the reproducing layer can be enlarged and reproduced. The magnetic domains transferred to the domain enlarging and reproducing layer are enlarged approximately up to the light beam spot size. This remarkably increases the strength of the reproduced signals. This technique is called the MAMMOS (magnetic amplifying magneto-optical system), which solves the problem with the magnetically induced super resolution technique about the C/N ratio of the signals reproduced from minute magnetic domains.
The MSR and MAMMOS involve utilizing the magnetic leakage fields leaking from the recorded magnetic domains in the recording layer of a magneto-optical recording medium to transfer the information on this layer to the reproducing layer of the medium, and thereafter reading out the information from the reproducing layer. However, it has been found that, under such a method or system, if the recording layer of a magneto-optical recording medium includes a continuous recorded magnetic domain (record mark) 227 consisting of two or more bits of information as shown in FIG. 2, the signal reproduced from the domain 227 is unstable in comparison with that from a shortest recorded magnetic domain which is present independently in the recording layer. This is conceived to be due to the difference in magnitude between the magnetic leakage fields at end portions and a middle portion of a continuous record mark formed on the recording layer. In particular, the magnetic leakage field leaking from the middle portion is weaker than that leaking from each of the end portions. It is consequently difficult to transfer the information on a middle portion of a continuous record mark to the reproducing layer. Therefore, in order to reproduce the information from a shortest record mark and the information from a continuous record mark, it is necessary to strictly control the power of the reproducing magnetic field and the power of the reproducing light beam, causing the problem that the reproducing power margins become narrower.
Japanese Patent Application Laid-Open No. 2-101659 discloses a method of recording binary information by pairing minimum record units and making the units of any pair different in magnetization. However, this Japanese publication does not show recording with a continuous recorded magnetic domain divided into magnetic domains magnetized in alternate directions.
In view of the problems with the foregoing prior art, it is an object of the present invention to provide a recording method making it possible to form, on the recording layer (of a magneto-optical recording medium), record marks which can be transferred reliably or securely to the reproducing layer (of the medium) during reproduction of information regardless of mark lengths of the marks.
It is another object of the invention to provide a recorder which can record continuous record marks having stable reproducing characteristics on the recording layer of a magneto-optical recording medium.
In accordance with a first aspect of the invention, a recording method is provided for recording information on a magneto-optical recording medium by radiating a recording light beam onto the medium while applying a magnetic field in a recording direction to the medium, the method comprising:
assigning one bit of information to a combination of a magnetic domain magnetized in the recording direction and a magnetic domain magnetized in the direction opposite to the recording direction, or to a combination of two magnetic domains magnetized in the direction opposite to the recording direction; and
adjusting sizes of the domains in such a manner that, when two or more bits of information each corresponding to the combination of the magnetic domain magnetized in the recording direction and the magnetic domain magnetized in the opposite direction are recorded in series, the domain magnetized in the recording direction for the first bit of the two or more bits is shorter than the domain magnetized in the recording direction for a bit contained in other bits than the first bit of the two or more bits.
A conventional method for recording information on a magneto-optical recording medium involves magnetizing the recording layer of the medium in one direction (initializing direction), and reversing the direction of magnetization of a magnetic domain in the recording layer, depending on whether the information is either binary xe2x80x9c0xe2x80x9d or binary xe2x80x9c1xe2x80x9d. If record information is defined as binary xe2x80x9c1xe2x80x9d, a long magnetic mark 227 as shown in FIG. 2 will be formed on the recording layer for a series of two or more bits of record data, such as xe2x80x9c1111xe2x80x9d. As stated above in the description of the related art section, there is a difference in strength between the magnetic leakage fields at a middle portion 228 and end portions 229 of the long mark 227. This results in a difference between the optimum power for reproducing the information positioned in the middle portion 228 and the optimum power for reproducing the information positioned in the end portions 229 or the information of a shortest mark existing independently on the recording layer. It is therefore difficult to reproduce information with the same reproducing power.
As shown in FIG. 3, the recording method according to the invention includes assigning one of binary xe2x80x9c1xe2x80x9d and binary xe2x80x9c0xe2x80x9d (for example, xe2x80x9c1xe2x80x9d) to a combination of a magnetic domain 221 magnetized in the recording direction (upward in FIG. 3) and a magnetic domain 222 magnetized in the opposite direction (downward in FIG. 3), like a pattern of magnetic domains 223, and assigning the other of binary xe2x80x9c1xe2x80x9d and binary xe2x80x9c0xe2x80x9d (for example, xe2x80x9c0xe2x80x9d) to a combination of two magnetic domains magnetized in the direction opposite to the recording direction, like a pattern of magnetic domains 224.
The recording direction represents one of the upward and downward directions perpendicular to the surfaces of the recording layer of a magneto-optical recording medium. Herein, for convenience, the recording direction is defined as the upward direction as shown in FIGS. 2-4. Herein, for convenience, an xe2x80x9c↑ magnetic domainxe2x80x9d represents a magnetic domain magnetized in the recording direction, while a xe2x80x9c⇓ magnetic domainxe2x80x9d represents a magnetic domain magnetized in the opposite direction (initializing or erasing direction). Herein, for convenience, binary xe2x80x9c1xe2x80x9d represents record information.
Under the recording method according to the first aspect of the invention, a piece of continuous record information, such as xe2x80x9c1111xe2x80x9d, is formed as a domain pattern consisting of one or more ↑ magnetic domains and one or more ⇓ magnetic domains (xe2x80x9c↑ ⇓ ↑ ⇓ ↑ ⇓ ↑ ⇓xe2x80x9d). In other words, even in the case of a piece of continuous record information being recorded, the or every (minute) magnetic domain magnetized in the recording direction adjoins the or a magnetic domain magnetized in the opposite direction. Consequently, whatever domain pattern includes magnetic domains corresponding to record information, the magnetic leakage fields leaking from these domains are equal in magnitude. Therefore, independent of the length of the continuous record information, it is possible to stably transfer the magnetizing information on the recording layer to the reproducing layer. Herein, one bit is a minimum unit of binary information, which may be a train of binary data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. Herein, the reproduction of information involves reading out bits of binary information one by one.
Under the recording method according to the invention, the recordation of information on the recording layer (of the recording medium) may involve converting each bit of binary information xe2x80x9c1xe2x80x9d into a train of codes xe2x80x9c10xe2x80x9d, converting each bit of binary information xe2x80x9c0xe2x80x9d into a train of codes xe2x80x9c00xe2x80x9d, and modulating at least one of the recording light beam and the recording magnetic field with the code trains. Specifically, record data bits xe2x80x9c1011xe2x80x9d may be converted into a code train xe2x80x9c10001010xe2x80x9d, which may be the basis for generating a record signal having a waveform as shown in FIG. 9. The magnetic field may be modulated with the record signal and applied.
In this invention, L1 is defined as the length of the ↑ magnetic domain, which is magnetized in the recording direction, for each bit of record information. On the other hand, L2 is defined as the length of the ⇓ magnetic domain, which is magnetized in the opposite direction, for each bit of record information. It is preferable that the ratio L2/L1 range between 0.1 and 0.9. Herein, the length of a magnetic domain means the circumferential length of the domain in the direction of the track of the recording medium. If the ratio of the length L2 of the ⇓ magnetic domain to the length L1 of the ↑ magnetic domain is 0.1 or higher, it is possible to stably generate magnetic leakage fields from the domains. If this ratio L2/L1 is 0.9 or lower, it is possible to obtain stable reproducing characteristics when continuous record information is recorded.
In general, the magnitude of the magnetic leakage field leaking from an ↑ magnetic domain depends on the sizes of this domain and the magnetic domain or domains adjoining it. Consequently, the magnetic leakage field leaking from the ↑ magnetic domain for the first bit of a piece of record information is greater in magnitude, due to the two ⇓ magnetic domains adjoining to this domain, than the magnetic leakage field leaking from the ↑ magnetic domain for the second bit of the information. Therefore, when two or more bits of continuous record information are recorded, it is preferable that the sizes of the magnetic domains be adjusted in such a manner that the ↑ magnetic domain for the first bit of the information is shorter than the ⇓ magnetic domain or domains for the other bit or bits, in order that the magnetic leakage fields leaking from the magnetic domains for the first and the other bits are uniform or equal.
In accordance with a second aspect of the invention, a recording method is provided for recording information on a magneto-optical recording medium by radiating a recording light beam onto a recording layer of the medium while applying a magnetic field in a recording direction to the layer, the method comprising:
modulating at least one of the recording light beam and the recording magnetic field in such a manner that an area of the recording layer which represents one bit of binary information which is one of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d consists of a magnetic domain magnetized in the recording direction and a magnetic domain magnetized in the direction opposite to the recording direction, and in such a manner that an area of the recording layer which represents one bit of binary information which is the other of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d consists of two magnetic domains magnetized in the direction opposite to the recording direction; and
adjusting sizes of recorded magnetic domains in such a manner that, when the magnetic domain magnetized in the recording direction has a length L1 while the magnetic domain magnetized in the opposite direction has a length L2, the ratio L2/L1 ranges between 0.1 and 0.9.
Under the recording method according to the second aspect of the invention, at least one of the recording light beam and the recording magnetic field is modulated in such a manner that the area representing a bit of binary information xe2x80x9c1xe2x80x9d is formed as a domain pattern (xe2x80x9c↑ ⇓xe2x80x9d) consisting of an ↑ magnetic domain and a ⇓ magnetic domain, and that the area representing a bit of binary information xe2x80x9c0xe2x80x9d is formed as a domain pattern (xe2x80x9c⇓ ⇓xe2x80x9d) consisting of two ⇓ magnetic domains. L1 may represent the length of the magnetic domain magnetized in the recording direction (↑ magnetic domain) out of the two domains (↑ magnetic domain and ⇓ magnetic domain) forming one bit of record information, while L2 may represent the length of the magnetic domain magnetized in the opposite direction (⇓ magnetic domain) out of the two domains. The ratio L2/L1 is adjusted to range between 0.1 and 0.9. Consequently, even if a piece of continuous record information is formed on the recording layer, it is possible to stably transfer the magnetizing information on the recording layer to the reproducing layer, independent of the length of the continuous record information, as substantially stated above in the description of the first aspect of the invention.
In accordance with a third aspect of the invention, a recording method is provided for recording information by forming magnetic domains magnetized in different directions in a recording layer of a magneto-optical recording medium, the method comprising:
assigning one of binary xe2x80x9c1xe2x80x9d and binary xe2x80x9c0xe2x80x9d to a domain train consisting of two magnetic domains magnetized in opposite directions;
assigning the other of binary xe2x80x9c1xe2x80x9d and binary xe2x80x9c0xe2x80x9d to a domain train consisting of two magnetic domains magnetized in the same direction; and
recording information in such a manner that the ratio in length of one of the two magnetic domains magnetized in opposite directions to the other ranges between 0.1 and 0.9.
Under the recording method according to the third aspect of the invention, the domain train consisting of two magnetic domains magnetized in opposite directions is assigned to one of binary xe2x80x9c1xe2x80x9d and binary xe2x80x9c0xe2x80x9d, which may be xe2x80x9c1xe2x80x9d, while the domain train consisting of two magnetic domains magnetized in the same direction is assigned to the other of binary xe2x80x9c1xe2x80x9d and binary xe2x80x9c0xe2x80x9d, which may be xe2x80x9c0xe2x80x9d. For example, an ↑ magnetic domain may represent a magnetic domain magnetized in a recording direction, while a ⇓ magnetic domain may represent a magnetic domain magnetized in the direction opposite to the recording direction. In this case, a domain train ↑ ⇓ consisting of an ↑ magnetic domain and a magnetic domain is assigned to binary xe2x80x9c1xe2x80x9d, while a domain train ⇓ ⇓ consisting of two ⇓ magnetic domains is assigned to binary xe2x80x9c0xe2x80x9d. The ratio in length of one of the two magnetic domains magnetized in opposite directions to the other is adjusted to range between 0.1 and 0.9. In the first aspect of the invention, binary xe2x80x9c1xe2x80x9d may correspond to one of the bits of binary information, while binary xe2x80x9c0xe2x80x9d may correspond to the other. This results in similar effects being obtained in the first and third aspects.
In accordance with a fourth aspect of the invention, a recording method is provided for recording information on a recording layer of a magneto-optical recording medium by radiating a recording light beam onto the medium while applying a magnetic field in a recording direction to the medium, the method comprising:
recording two or more consecutive bits of record information on the recording layer in such a manner that at least one bit of the consecutive bits is composed of a magnetic domain magnetized in the recording direction and a magnetic domain magnetized in the direction opposite to the recording direction.
In accordance with a fifth aspect of the invention, a recording method is provided for recording information on a recording layer of a magneto-optical recording medium by radiating a recording light beam onto the medium while applying a magnetic field in a recording direction to the medium, the method comprising:
modulating at least one of the recording magnetic field and the recording light beam in such a manner that, when two or more consecutive bits of record information are recorded, a magnetic domain magnetized in the direction opposite to the recording direction is formed in a recording area of the recording layer which corresponds to one bit of record information.
Under the recording methods according to the fourth and fifth aspects of the invention, as shown in FIG. 4, the recordation of a bit of single record information involves forming this information as only an isolated recorded magnetic domain 441 magnetized in the recording direction. As also shown in FIG. 4, the recordation of two or more consecutive bits of record information involves forming each of them as a recorded magnetic domain 442 magnetized in the recording direction and a recorded magnetic domain 443 magnetized in the opposite direction. This makes it possible to obtain uniform magnetic leakage fields even from the continuous record information, independent of the positions of the magnetic domains forming this information. Therefore, the reproducing power margin is not narrowed due to the length of the consecutive recorded magnetic domains.
In accordance with a sixth aspect of the invention, a recorder is provided for recording information on a magneto-optical recording medium, the recorder comprising:
a light source which irradiates the recording medium with a recording light beam;
a magnetic field applying device which applies a recording magnetic field; and
a modulator for modulating record data represented by a data train with a data bit in a recording direction and/or a data bit in the direction opposite to the recording direction, in such a manner that a data bit in the direction opposite to the recording direction is interposed between the bits of the train;
the modulator outputting a signal with which to modulate at least one of the recording light beam and the recording magnetic field.
Thus, if record data are represented by a data train consisting of a data bit xe2x80x9c1xe2x80x9d in the recording direction and a data bit xe2x80x9c0xe2x80x9d in the opposite direction, the modulator of the recorder according to the invention modulates the data in such a manner that a data bit in the direction opposite to the recording direction is interposed between the bits of the train. The record data are recorded with at least one of the recording light beam and magnetic field modulated with the modulated data. In other words, it is possible to convert the data bit xe2x80x9c1xe2x80x9d as record information into a code train xe2x80x9c10xe2x80x9d, convert each data bit xe2x80x9c0xe2x80x9d into a code train xe2x80x9c00xe2x80x9d, and modulate at least one of the recording light beam and the recording magnetic field with the code trains. By using this recorder, it is possible to implement the recording methods according to the first to fifth aspects of the invention, enabling uniform magnetic leakage fields to be generated independent of the length of the continuous record information and the positions of the recorded magnetic domains. Therefore, the recorder according to the invention is suitable for recording on a magneto-optical recording medium including a recording layer and a reproducing layer to which the magnetizing information on the recording layer can be transferred.
In accordance with a seventh aspect of the invention, a recording method is provided for recording information on a magneto-optical recording medium including a recording layer by radiating a recording light beam onto the medium while applying a magnetic field in a recording direction to the medium, the method comprising:
recording two or more consecutive bits of record information in such a manner that a recording area of the recording layer which corresponds to the consecutive bits of record information includes at least a magnetic domain magnetized in the direction opposite to the recording direction.
The recording method according to the seventh aspect of the invention involves recording a piece of continuous information, which is a train of magnetic domains (or continuous record mark), by dividing the continuous record mark in such a manner that the mark includes at least one magnetic domain (or record mark) magnetized in the direction opposite to the direction in which the domain train is magnetized. In order for a continuous record mark representing two or more consecutive bits of record information to include a magnetic domain magnetized in the opposite direction, for example, the record signal based on the record information may be modulated. The (mark) length of the magnetic domain out of the consecutive magnetic domains which is magnetized in the opposite direction is not limited to the length of one bit, but may be any length.
In accordance with an eighth aspect of the invention, a recording method is provided for recording information on a recording layer of a magneto-optical recording medium by radiating a recording light beam onto the medium while applying a magnetic field in a recording direction to the medium, the method comprising:
applying a magnetic field having a magnetic field strength H1 in the recording direction when recording a record mark having a mark length A on the recording layer;
applying a magnetic field having a magnetic field strength H2 (H2xe2x89xa0H1) in the recording direction when recording a record mark having a mark length B (Bxe2x89xa0A) on the recording layer.
The recording method according to the eighth aspect of the invention involves modulating the magnetic field strength of a recording magnetic field with the length of a record mark for formation in the recording layer. Specifically, the recording magnetic field having the magnetic field strength H1 is applied when the record mark having the mark length A is recorded, and the recording magnetic field having the magnetic field strength H2, which differs from the strength H1, is applied when the record mark having the mark length B is recorded. For example, if the record mark having the mark length A is a shortest record mark, the mark is formed in the recording layer by applying the recording magnetic field having the magnetic field strength H1 which is the same as conventionally. The record mark having the mark length B (a continuous record mark) is formed in the recording layer by applying the recording magnetic field having the magnetic field strength H2 which is lower than the strength H1. The record mark having the mark length B and recorded with the lower magnetic field strength is a mixture of magnetic domains in the recording and opposite directions (that is, the magnetic domains in the recording direction are distributed). It is possible to obtain magnetic leakage fields with sufficient magnetic field strength anywhere from a mixture of magnetic domains in the recording and opposite directions. This makes it possible to obtain sufficient magnetic leakage fields from a middle portion as well as the front and rear edges of a continuous record mark. Consequently, the magnetic leakage fields can be uniform over the entire continuous record mark.